The present invention relates generally to drill bits, and more specifically relates to drill bits for earth boring, which includes cutters comprising an array of discrete cutting elements.
It is known in the art that certain earth formations are more susceptible to being bored with bits having large cutters thereon, usually so-called "plastic" or "gumbo" formations, where small cutters get mud-bound with drilling mud and the bit consequently "balls up", slowing or stopping forward progress of the well bore. Large unitary cutters, large being referred to herein as those of 3/4" diameter and above, are generally more expensive than their smaller counterparts, and present problems of their own when mounted on a bit face. Specifically, when polycrystalline diamond compact ("PDC") cutters are brazed or otherwise metallurgically bonded to a support or carrier surface on a bit face, the differing coefficients of thermal expansion between the PDC substrate material and that of the support or carrier subject the PDC to a large, permanent residual stress when the braze cools, thus rendering the PDC more susceptible to fracture upon impact with the formation and/or fracture at the braze or metallurgical bond line. Moreover, as alluded to above, PDC's must be bonded to the bit body or to a carrier, which itself is secured on the bit face after the furnacing of a matrix-type bit, which usually comprises a matrix of tungsten carbide powder bonded together by a copper-based binder alloy. The method of producing such a bit is well known in the art, and comprises manufacturing a mold or "boat" of graphite, ceramic or other material which possesses on its interior the characteristics of the bit face to be produced, these characteristics being milled or otherwise cut or molded therein; filling the mold with a tungsten carbide or other suitable powder, placing beads of a binder alloy in the mold as well as flux; and furnacing the bit at a temperature high enough to infiltrate the powder with the melted binder alloy.
If, as noted above, one wishes to use PDC cutters on the bit, it is necessary to bond them to the bit face after furnacing, as the furnacing temperature, generally in excess of 1070.degree. C., will thermally degrade PDC's into a fragile, brittle and/or relatively soft state, making them useless as cutters. It is known to furnace natural diamonds directly into a bit body, as natural diamonds have a thermal stability suitable for such an operation. Similarly, there exist on the market so-called "thermally stable" polycrystalline diamond compact products ("TSP's") which can survive furnacing without significant degradation. Two types of TSP's are on the market today, leached products, where most of the non-diamond material in the compact has been removed, and unleached products, where the non-diamond material in the compact possesses similar thermal expansion characteristics to the diamond and does not degrade the diamond at temperatures up to 1200.degree. C. In either case, these TSP's may be furnaced into the bit, providing a cutter-laden bit in a single operation. Affixation of the TSP cutters to the bit face may be enhanced by coating them with metal as is known in the art, to provide a chemical (metallurgical) bond between the bit matrix and cutter. One exemplary apparatus and method for coating TSP elements is described in U.S. Pat. No. 4,943,488, issued on Jul. 24, 1990 and assigned to the designee of this application. The specification of U.S. Pat. No. 4,943,488 is incorporated herein by this reference.
In some soft, plastic formations, there are stringers of harder, more abrasive rock, or a bit may have to drill through both soft and hard, abrasive rock in close succession without being pulled from the well bore. Bits having several types of cutting elements for cutting different types of formations are known; see for example, U.S. Pat. No. 4,512,426 to Bidegaray, assigned to Eastman Christensen Company. Using TSP elements in conjunction with PDC's is known. One such bit design uses PDC cutters in combination with cutters comprising mosaic-like arrays of small, triangular-faced polyhedral TSP's, each array simulating a larger unitary cutter. Such bits are sold by the Eastman Christensen Company of Salt Lake City, Utah, U.S.A., as the Mosaic.TM. series of bits. The type of cutter utilized on the aforesaid bits is described in U.S. Pat. No. 4,726,718, assigned to Eastman Christensen Company and the bonding of the TSP's into an array may be enhanced by the coating process of the above-referenced U.S. Pat. No. 4,943,488.
Planar TSP cutters up to at least 1.5 inches in diameter are available from DeBeers under the trade-name "Syndax 3." Such cutters are not readily bonded during infiltration to matrix-type bits and substantial residual stresses will result upon cooling the bit due to the difference in thermal expansion of the TSP and the bit matrix. Moreover, large single pieces provide less geometric flexibility.
It has been proposed to fabricate very large TSP array cutters, and even entire cutter blades extending from the gage of the bit to the center of the bit face. See, for example, U.S. Pat. No. 4,913,247, issued on Apr. 3, 1990, in the name of Mark L. Jones, and assigned to Eastman Christensen Company. Such TSP-array cutter bits would not only provide a large cutting surface for plastic formations, but be abrasion-resistant so as to better survive stringers, in addition to being furnaceable into the bit.
Clearly, it is desirable to produce a bit having large cutting surfaces at reasonable cost and without the aforementioned thermal stress problems. Merely enlarging the array of small TSP elements, such as is suggested in the Jones application, was believed to be a solution, the theory being that a plurality of small TSP elements would economically form a large, predominantly-diamond cutting surface without being detrimentally affected by the thermal stress associated with a large, unitary cutter. However, it has been discovered that this thermal stress problem pervades even a TSP array, in that bits, incorporating large TSP arrays, have encountered delamination of the entire layer of TSP elements, both before and during drilling, due to the stress between the TSP elements and the bit matrix. The coating method of the above-referenced Sung and Chen application, while enhancing the diamond to matrix bond, actually aggravates the stress problem due to the strength of the diamond to matrix bond. In fact, instances of diamond fracture instead of bond fracture have been experienced under stress.
Stress between the TSP elements and the bit matrix is believed to occur during cooling of the bit after furnacing as a result of the different thermal expansion rates of the TSP and the matrix. Stress cracks are generally parallel to the TSP/matrix interface, and may later intersect with cracks in the cutter surface caused by impact stresses experienced during drilling, thereby resulting in premature cutter loss from the bit.
Accordingly, there is a need for a cutter configuration which can provide large cutting surfaces without the self-destructive tendencies of the large cutters and cutter arrays of the prior art.